Glucose
a simple monosaccharide sugar, is one of the most
important carbohydrates and is used as a source
of energy in animals and plants. Glucose is one
of the main products of photosynthesis and starts
respiration. The natural form (D-glucose) is also
referred to as dextrose, especially
in the food industry.

A Haworth projection
representation of the structure of glucose

Glucose (C6H12O6)
is a hexose -- a monosaccharide containing six
carbon atoms. Glucose is an aldehyde (contains
a -CHO group). Five of the carbons plus an oxygen
atom form a loop called a "pyranose ring", the
most stable form for six-carbon aldoses. In this
ring, each carbon is linked to hydroxyl and hydrogen
side groups with the exception of the fifth atom,
which links to a 6th carbon atom outside the ring,
forming a CH2OH group. This ring structure
exists in equilibrium with a more reactive acyclic
form, which makes up 0.0026% at pH 7.

Glucose is a ubiquitous fuel
in biology. We can speculate on the reasons why
glucose, and not another monosaccharide such as
fructose, is so widely used. Glucose can form
from formaldehyde under abiotic conditions, so
it may well have been available to primitive biochemical
systems. Probably more important to advanced life
is the low tendency of glucose, by comparison
to other hexose sugars, to nonspecifically react
with the amino groups of proteins. This reaction
(glycosylation) reduces or destroys the function
of many enzymes. The low rate of glycosylation
is due to glucose's preference for the less reactive
cyclic isomer. In respiration, through a series
of enzyme-catalysed reactions, glucose is oxidized
to eventually to form carbon dioxide and water,
yielding energy, mostly in the form of ATP.

Chemically joined together,
glucose and fructose form sucrose. Starch, cellulose, and glycogen are common glucose
polymers (polysaccharides).

The older name dextrose arose
because a solution of D-glucose rotates polarised
light towards the right. In the same vein D-fructose was called "levulose"
because a solution of levulose rotates polarised
light to the left.

Isomerism

There are two enantiomers
(mirror-image isomers) of the sugar -- D-glucose
and L-glucose, but in living organisms only the
D-isomer is found. The ring structure may form
in two different ways, yielding ± (alpha)
glucose and ² (beta) glucose. Structurally,
they differ in the orientation of the hydroxyl
group linked to the first carbon in the ring.
The ± form has the hydroxyl group "below"
the hydrogen (as the molecule is conventionally
drawn, as in the figure above), while the ²
form has the hydroxyl group "above" the hydrogen.
These two forms interconvert on a timescale of
hours in aqueous solution, to a ratio of ±:²
36:64, in a process called mutarotation.

D-Glucose has the same configuration
at its penultimate carbon as D-glyceraldehyde.

Synthesis

The product of photosynthesis
in plants and some prokaryotes.

Formed in the liver
by the breakdown of glycogen stores (Glucose
polymers).